U.S. patent application number 11/870020 was filed with the patent office on 2008-04-17 for extreme ultra violet light source apparatus.
This patent application is currently assigned to KOMATSU LTD.. Invention is credited to Georg SOUMAGNE, Akira SUMITANI, Yoshifumi UENO, Osamu WAKABAYASHI.
Application Number | 20080087840 11/870020 |
Document ID | / |
Family ID | 39302312 |
Filed Date | 2008-04-17 |
United States Patent
Application |
20080087840 |
Kind Code |
A1 |
UENO; Yoshifumi ; et
al. |
April 17, 2008 |
EXTREME ULTRA VIOLET LIGHT SOURCE APPARATUS
Abstract
An extreme ultra violet light source apparatus having relatively
high output for exposure, in which debris are suppressed to be
produced as much as possible in stead of disposing debris that has
been once produced. The extreme ultra violet light source apparatus
includes: a chamber in which extreme ultra violet light is
generated; a target supply unit for supplying solid tin or lithium
as a target to a predetermined position within the chamber; a
CO.sub.2 laser for applying a laser beam based on pulse operation
to the target supplied by the target supply unit so as to generate
plasma; and a collector mirror having a multilayer film on a
reflecting surface thereof, for collecting the extreme ultra violet
light radiated from the plasma to output the extreme ultra violet
light.
Inventors: |
UENO; Yoshifumi; (Hiratsuka,
JP) ; SOUMAGNE; Georg; (Kamakura, JP) ;
SUMITANI; Akira; (Isehara, JP) ; WAKABAYASHI;
Osamu; (Hiratsuka, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W., SUITE 800
WASHINGTON
DC
20006-1021
US
|
Assignee: |
KOMATSU LTD.
Tokyo
JP
GIGAPHOTON INC.
Tokyo
JP
|
Family ID: |
39302312 |
Appl. No.: |
11/870020 |
Filed: |
October 10, 2007 |
Current U.S.
Class: |
250/396ML ;
250/504R |
Current CPC
Class: |
H05G 2/001 20130101;
G03F 7/70033 20130101; B82Y 10/00 20130101 |
Class at
Publication: |
250/396ML ;
250/504.R |
International
Class: |
H05G 2/00 20060101
H05G002/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 16, 2006 |
JP |
2006-281186 |
Claims
1. An extreme ultra violet light source apparatus which generates
extreme ultra violet light by applying a laser beam to a target,
said apparatus comprising: a chamber in which extreme ultra violet
light is generated; a target supply unit which supplies one of
solid tin and solid lithium as a target to a predetermined position
within said chamber; a CO.sub.2 laser which applies a laser beam
based on pulse operation to the target supplied by said target
supply unit so as to generate plasma; and a collector mirror which
has a multilayer film on a reflecting surface thereof and collects
the extreme ultra violet light radiated from said plasma to output
the extreme ultra violet light.
2. The extreme ultra violet light source apparatus according to
claim 1, wherein said CO.sub.2 laser applies a laser beam having
intensity of 3.times.10.sup.9 W/cm.sup.2 to 5.times.10.sup.10
W/cm.sup.2 to the target supplied by said target supply unit.
3. The extreme ultra violet light source apparatus according to
claim 1, wherein said CO.sub.2 laser applies a laser beam having a
pulse width of 10 ns to 15 ns to the target supplied by said target
supply unit.
4. The extreme ultra violet light source apparatus according to
claim 2, wherein said CO.sub.2 laser applies a laser beam having a
pulse width of 10 ns to 15 ns to the target supplied by said target
supply unit.
5. The extreme ultra violet light source apparatus according to
claim 1, further comprising: a first detector which detects an
amount of neutral particles emitted from said plasma; a second
detector which detects an amount of ions emitted from said plasma;
a control unit which obtains an amount of neutral particles
deposited on the reflecting surface of said collector mirror based
on detection result of said first detector and an amount of the
multilayer film worn away from the reflecting surface of said
collector mirror based on detection result of said second detector,
and thereby, adjusts a pulse width and/or energy of the laser beam
radiated from said CO.sub.2 laser such that the amount of the
neutral particles deposited on the reflecting surface of said
collector mirror is not larger than the amount of the multilayer
film worn away from the reflecting surface of said collector
mirror.
6. The extreme ultra violet light source apparatus according to
claim 1, wherein said target supply unit supplies the target to the
predetermined position within said chamber while adjusting a
position of the target.
7. The extreme ultra violet light source apparatus according to
claim 1, wherein said target supply unit includes a feed mechanism
including a feed reel and a motor which feed a wire-like or
tape-like target, and a take-up mechanism including a take-up reel
and a motor which take up the target.
8. The extreme ultra violet light source apparatus according to
claim 1, wherein said target supply unit includes two rod transport
mechanisms which transport a rod-like target by holding both ends
of the target and moving the target in parallel while turning the
target.
9. The extreme ultra violet light source apparatus according to
claim 1, wherein said target supply unit supplies a solid
droplet-shaped to the predetermined position within said chamber
via a target nozzle while adjusting a fall position of the
target.
10. The extreme ultra violet light source apparatus according to
claim 1, wherein said target supply unit includes a plate
turning/moving mechanism which turns a plate-like target and/or
moves the target in parallel.
11. The extreme ultra violet light source apparatus according to
claim 1, further comprising: a magnetic field generating unit which
generates a magnetic field when current is supplied thereto so as
to trap charged particles emitted from said plasma.
12. The extreme ultra violet light source apparatus according to
claim 11, wherein said magnetic field generating unit includes one
of a pair of mirror coils and a baseball coil.
13. The extreme ultra violet light source apparatus according to
claim 11, further comprising: a current supply unit which supplies
current to said magnetic field generating unit such that said
magnetic field generating unit generates a stationary magnetic
field.
14. The extreme ultra violet light source apparatus according to
claim 11, further comprising: a current supply unit which supplies
current to said magnetic field generating unit; and a control unit
which controls said current supply unit to cause said magnetic
field generating unit to periodically generate a pulse magnetic
field.
15. The extreme ultra violet light source apparatus according to
claim 11, further comprising: a current supply unit which supplies
current to said magnetic field generating unit; a detector which
detects a change in thickness of a measurement film due to neutral
particles and ions emitted from said plasma; and a control unit
which controls said current supply unit based on a detection result
of said detector such that an amount of neutral particles deposited
on the measurement film is not larger than an amount of components
worn away from the measurement film.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an extreme ultra violet
(EUV) light source apparatus to be used as a light source of
exposure equipment.
[0003] 2. Description of a Related Art
[0004] Recent years, photolithography has made rapid progress to
finer fabrication with finer semiconductor processes. In the next
generation, microfabrication of 100 nm to 70 nm, further,
microfabrication of 50 nm or less will be required. Accordingly, in
order to fulfill the requirement for microfabrication of 50 nm or
less, for example, exposure equipment is expected to be developed
by combining an EUV light source generating EUV light with a
wavelength of about 13 nm and reduced projection reflective
optics.
[0005] The EUV light sources include three kinds, namely, an LPP
(laser produced plasma) light source using plasma generated by
applying a laser beam to a target, a DPP (discharge produced
plasma) light source using plasma generated by electric discharge,
and an SR (synchrotron radiation) light source using orbital
radiation. Among them, the LPP light source has advantages that
extremely high intensity near black body radiation can be obtained
because plasma density can be considerably made larger, that light
emission of only the necessary waveband can be performed by
selecting the target material, that an extremely large collection
solid angle of 2.pi. steradian can be ensured because of a point
light source having substantially an isotropic angle distribution
and no structure surrounding the light source such as electrodes,
and so on. Therefore, the LPP light source is thought to be
predominant as a light source for EUV lithography requiring power
of several tens of watts or more.
[0006] Here, a principle of generating EUV light with the LPP
system will be explained. When a laser beam is applied to a target
material supplied into a vacuum chamber, the target material is
excited and plasmarized. Various wavelength components including
EUV light are radiated from the plasma. Then, the EUV light is
reflected and collected by using an EUV collector mirror that
selectively reflects a desired wavelength component (e.g., a
component having a wavelength of 13.5 nm), and outputted to an
exposure unit. For the purpose, a multilayer film in which thin
films of molybdenum (Mo) and thin films of silicon (Si) are
alternately stacked (Mo/Si multilayer film), for example, is formed
on the reflecting surface of the EUV collector mirror.
[0007] In the LPP EUV light source apparatus, the influence by
neutral particles and ions emitted from plasma is problematic
especially when a solid target is used. Since the EUV collector
mirror is located near the plasma, the neutral particles emitted
from the plasma are deposited on the reflecting surface of the EUV
collector mirror and reduce the reflectance of the mirror. On the
other hand, the ions emitted from the plasma wears away the
multilayer film formed on the reflecting surface of the EUV
collector mirror (the shaving is also defined as "sputtering" in
the present application). The scattered materials from the plasma
including neutral particles and ions, and the remains of the target
materials are called debris.
[0008] In order to maintain the high reflectance, the high flatness
of about 0.2 nm (rms), for example, is required for the EUV
collector mirror, and thus, the mirror is very expensive.
Accordingly, the longer life of the EUV collector mirror is desired
in view of reduction in operation costs of the exposure equipment,
reduction in maintenance time, and so on. The mirror life in the
EUV light source apparatus for exposure is defined as a period
until the reflectance decreases 10%, for example, and at least
one-year life is required.
[0009] In order to fulfill the requirement, International
Publication WO 02/46839 A2 discloses a special liquid droplet
targets that are irradiated by a high power laser and are
plasmarized to form a point source of EUV, XUV or X-ray. As various
types of liquid droplet targets, solutions of metallic chloride,
metallic bromide, and soon, or nano-sized particles (e.g.,
aluminum, bismuth, or the like) in solutions (e.g., water or oil)
having a melting temperature lower than the melting temperature of
some of the constituent metals are used. Since the target is in the
form of droplets, the sufficient distance from the nozzle supplying
the target to the plasma can be secured. Further, by restricting
the mass of the target, the debris can be suppressed.
[0010] However, when the mass of the target is restricted to the
degree that no debris is produced at all, various technical
problems may occur and the apparatus may be complicated. For
example, clogging occurs in the nozzle for supplying the target,
and the EUV conversion efficiency (CE) decreases according to the
reduction in target size. On the other hand, for optimization of
the target size and density in order to improve the EUV conversion
efficiency, it is necessary to expand the droplets by using
pre-pulse laser.
[0011] Further, International Publication WO 2004/092693 A2
discloses a method and apparatus for debris removal from a
reflecting surface of an EUV collector mirror in an EUV light
source. Especially, in FIGS. 2A and 2B of WO 2004/092693 A2, a
debris shield including plural thin plates that define radially
extending optical paths is shown. By locating the debris shield
between a point light source formed at the plasma center and the
EUV collector mirror, the debris deposited on the reflecting
surface of the EUV collector mirror can be reduced.
[0012] However, since the debris shield is exposed to the plasma,
the thin plates of the debris shield is worn away by fast ions and
debris is produced, and the produced debris may be deposited on the
reflecting surface of the EUV collector mirror. In this case, the
debris shield itself becomes a debris source.
[0013] Furthermore, US Patent Application Publication US
2005/0279946 A1 discloses a system for protecting an internal EUV
light source component from ions generated at a plasma formation
site. In one aspect, the system may comprise a plurality of foil
plates and equipment for generating a magnetic field to deflect
ions into a surface of one of the foil plates. In another aspect,
an electrostatic grid may be positioned for interaction with ions
to reduce ion energy.
[0014] However, the method of guiding debris by using a magnetic
field or electric field is effective for ions, but not effective
for neutral particles.
[0015] When a metallic film is deposited on the reflecting surface
of the EUV collector mirror, EUV light is absorbed while making a
round trip through the metallic film.
[0016] Accordingly, when the optical transmittance of the metallic
film becomes about 95%, the reflectance of the EUV collector mirror
becomes about 90%. In order to hold the reduction in reflectance of
the EUV collector mirror within 10% with respect to EUV light
having a wavelength of 13.5 nm, the acceptable value of the amount
of deposition (thickness) of the metallic film on the reflecting
surface of the EUV collector mirror is a very little value such as
about 0.75 nm for tin (Sn) and about 5 nm for lithium (Li).
[0017] Therefore, the achievement of one-year mirror life is
considered to be very difficult in an EUV light source apparatus
for exposure having output of about 115 W to 180 W at the focusing
point only by restricting the mass of the target or providing a
debris shield.
SUMMARY OF THE INVENTION
[0018] The present invention has been achieved in view of the
above-mentioned problems. A purpose of the present invention is, in
an EUV light source apparatus having relatively high output for
exposure, not to dispose of debris that has been once produced, but
to suppress debris to be produced as much as possible.
[0019] In order to accomplish the above purpose, an extreme ultra
violet light source apparatus according to one aspect of the
present invention is an extreme ultra violet light source apparatus
which generates extreme ultra violet light by applying a laser beam
to a target, and the apparatus includes: a chamber in which extreme
ultra violet light is generated; a target supply unit which
supplies one of solid tin and solid lithium as a target to a
predetermined position within the chamber; a CO.sub.2 laser which
applies a laser beam based on pulse operation to the target
supplied by the target supply unit so as to generate plasma; and a
collector mirror which has a multilayer film on a reflecting
surface thereof and collects the extreme ultra violet light
radiated from the plasma to output the extreme ultra violet
light.
[0020] Here, it is desirable that the CO.sub.2 laser applies a
laser beam, which has intensity of 3.times.10.sup.9 W/cm.sup.2 to
5.times.10.sup.10 W/cm.sup.2 and/or a pulse width of ions to 15 ns,
to the target supplied by the target supply unit.
[0021] According to the present invention, production of debris can
be suppressed as much as possible by applying a laser beam having a
relatively long wavelength generated by the CO.sub.2 laser to the
target of solid tin or lithium.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a side view showing an internal structure of an
EUV light source apparatus according to the first embodiment of the
present invention;
[0023] FIG. 2 is a plan view showing the internal structure of the
EUV light source apparatus according to the first embodiment of the
present invention;
[0024] FIG. 3 shows wavelengths and critical density of a CO.sub.2
laser and an Nd:YAG laser;
[0025] FIG. 4 shows a state in which a laser beam is reflected from
the vicinity of the target;
[0026] FIG. 5 is a plan view showing an arrangement of detectors in
a laser beam application experiment;
[0027] FIG. 6 shows results of a laser beam application experiment
when a tin plate having a thickness of 1 mm is used as the
target;
[0028] FIG. 7 shows results of a laser beam application experiment
when a tin foil having a thickness of 15 .mu.m is used as the
target;
[0029] FIG. 8 shows a state in which an excitation laser beam is
applied to a wire target;
[0030] FIG. 9 shows a state in which the excitation laser beam is
applied to a tape target;
[0031] FIG. 10 shows a target supply unit for supplying the wire
target or tape target;
[0032] FIG. 11 shows a state in which the excitation laser beam is
applied to a rod target;
[0033] FIG. 12 shows a target supply unit for supplying the rod
target;
[0034] FIG. 13 is a side view showing an internal structure of the
EUV light source apparatus using a solid droplet target;
[0035] FIG. 14 is a plan view showing an internal structure of the
EUV light source apparatus using a plate target;
[0036] FIG. 15 shows a configuration of an EUV light source
apparatus according to the second embodiment of the present
invention;
[0037] FIG. 16 is a perspective view of electromagnet coils shown
in FIG. 15;
[0038] FIG. 17 is a diagram for explanation of a principle of
trapping ions by using a mirror magnetic field;
[0039] FIG. 18 shows tracks of ions when the interior of the EUV
light source apparatus shown in FIG. 15 is seen from above;
[0040] FIG. 19 is a perspective view of an electromagnet coil in an
EUV light source apparatus according to the third embodiment of the
present invention; and
[0041] FIG. 20 is a diagram for explanation of a principle of
trapping ions by using a baseball magnetic field.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] Hereinafter, preferred embodiments of the present invention
will be explained in detail by referring to the drawings. The same
reference numerals are assigned to the same component elements and
the description thereof will be omitted.
[0043] FIG. 1 is a side view showing an internal structure of an
EUV light source apparatus according to the first embodiment of the
present invention, and FIG. 2 is a plan view showing the internal
structure of the EUV light source apparatus according to the first
embodiment of the present invention. The EUV light source apparatus
according to the embodiment employs a laser produced plasma (LPP)
system of generating EUV light by applying a laser beam to a target
material for excitation.
[0044] As shown in FIGS. 1 and 2, the EUV light source apparatus
includes a vacuum chamber 10 in which EUV light is generated, a
target supply unit 11 that supplies a target 1, a driver laser 13
that generates an excitation laser beam 2 to be applied to the
target 1, a laser beam focusing optics 14 that collects the
excitation laser beam 2 generated by the driver laser 13, an EUV
collector mirror 15 that collects EUV light 4 emitted from plasma
3, which is generated when the excitation laser beam 2 is applied
to the target 1, to output the EUV light 4, a target collection
unit 16 that collects the target 1, a target circulation unit 17
that circulates the target 1, and a control unit 30 that controls
the entire EUV light source apparatus.
[0045] The vacuum chamber 10 is provided with a lead-in window 18
that leads in the excitation laser beam 2 and a lead-out window 19
that leads out the EUV light radiated from the plasma to an
exposure unit. The interior of the exposure unit is held in vacuum
or under reduced pressure as well as the interior of the vacuum
chamber 10. The target supply unit 11 includes a position
adjustment mechanism for adjusting the position of the target 1 to
be applied with the excitation laser beam 2, and supplies the
target 1 to a predetermined position of the vacuum chamber 10 while
adjusting the position of the target 1.
[0046] The driver laser 13 is a laser beam source capable of pulse
oscillation at a high repetition frequency (e.g., the pulse width
is about several nanoseconds to several tens of nanoseconds, and
the frequency is about 1 kHz to about 100 kHz). Further, the laser
beam focusing optics 14 includes at least one lens and/or at least
one mirror. The laser beam 2 collected by the laser beam focusing
optics 14 is applied at the predetermined position within the
vacuum chamber 10 to the target supplied by the target supply unit
11, and thereby, part of the target 1 is excited and plasmarized,
and various wavelength components are radiated from a light
emission point. Here, the light emission point refers to a position
where the plasma 3 is generated.
[0047] The EUV collector mirror 15 is a collection optics that
selectively reflects and collects a predetermined wavelength
component (e.g., EUV light near 13.5 nm) of the various wavelength
components radiated from the plasma 3. The EUV collector mirror 15
has a concave reflecting surface, and a multilayer film of
molybdenum (Mo) and silicon (Si) for selectively reflecting the EUV
light near 13.5 nm in wavelength, for example, is formed on the
reflecting surface. In FIG. 1, the EUV light is reflected to the
right by the EUV collector mirror 15, collected at an EUV
intermediate focusing point, and then, outputted to the exposure
unit. The collection optics of EUV light is not limited to the EUV
collector mirror 15 shown in FIG. 1, but the collection optics may
be configured by using plural optical components to be a reflection
optics for suppressing absorption of EUV light.
[0048] The target collection unit 16 includes a position adjustment
mechanism for adjusting the position of the target 1 to be applied
with the excitation laser beam 2, and located in a position facing
the target supply unit 11 with the light emission point in between.
The target collection unit 16 collects the targets that have not
been plasmarized. The collected targets may be returned to the
target supply unit 11 by the target circulation unit 17 to be
reused.
[0049] Furthermore, the EUV light source apparatus includes a
mirror damage detector 21 for detecting an amount of neutral
particles emitted from the plasma 3, an ion detector 22 for
detecting an amount of ions emitted from the plasma 3, and a
multilayer film mirror 23 and an EUV light detector 24 for
detecting the intensity of EUV light in the light emission point
not via the EUV collector mirror 15.
[0050] The mirror damage detector 21 includes a QCM (quartz crystal
microbalance), for example. The QCM is a sensor capable of
measuring the change in thickness of a sample film (a film for
measurement) of gold (Au) or the like formed on the sensor surface
based on the change in resonance frequency of a quartz crystal with
an accuracy within one angstrom in real time. An amount of neutral
particles deposited on the reflecting surface of the EUV collector
mirror (hereinafter, also referred to as "deposition amount") can
be obtained based on the change in thickness of the sample film
detected by the mirror damage detector 21.
[0051] The ion detector 22 includes a Faraday cup, for example. An
amount of multilayer film worn away from the reflecting surface of
the EUV collector mirror (hereinafter, also referred to as
"sputtering amount") can be obtained based on the amount of ions
detected by the ion detector 22.
[0052] On the multilayer mirror 23, a multilayer film of molybdenum
and silicon that has high reflectance to the wavelength near 13.5
nm, for example, is formed. The EUV light detector 24 includes a
zirconium (Zr) filter and a photodiode, for example. The zirconium
filter cuts light having a wavelength longer than 20 nm. The photo
diode outputs a detection signal corresponding to the intensity or
energy of incident light.
[0053] In the embodiment, a CO.sub.2 laser capable of generating
light having a relatively long wavelength is used as the driver
laser 13. Further, solid tin (Sn) or lithium (Li) is used as the
target 1. The reason is as follows.
[0054] Generally, it is known that, when a laser beam is applied to
a target and plasma is generated, the melting layer on the target
surface is suddenly boiled or the expansion pressure of the plasma
is applied to the target, and part of the melting target is
granulated and spouted (see Kobayashi et al., "Generation and
Control of Ablation Plasma 1 (Laser)", JORNAL OF PLASMA AND FUSION
RESEARCH, Vol. 76, No. 11 (November 2000), pp. 1145-1150).
[0055] Especially, in a plasma light source using a solid target,
there are a high-temperature low-density plasma region that
generates a radiant ray in a short wavelength range such as EUV
light and a low-temperature high-density plasma region that
generates no radiant ray in the short wavelength range. One of
them, the low-temperature high-density plasma region serves as a
heat source that generates a large amount of debris from the target
material after laser radiation. A melting layer is formed on the
surface of the target by the heat source and the melted metal is
spouted and scattered due to expansion pressure of the plasma, and
thus, debris is generated.
[0056] The process will be specifically explained. When a laser
beam is applied to the target material, the target material is
heated and ionized by the laser beam, and plasma is generated.
Then, the laser beam is absorbed by the plasma. The mechanism of
absorption of the laser beam by the plasma is an absorption
mechanism of a reversal process to the Bremsstrahlung or braking
radiation that emits electromagnetic wave (laser beam) when an
electron is accelerated in an electric field formed by ions, and
called inverse-Bremsstrahlung absorption. The
inverse-Bremsstrahlung absorption is the most basic absorption
mechanism occurring in laser generated plasma, and also called
classical absorption. The electron vibrating due to ad
high-frequency electric field causes energy absorption while
colliding with ions.
[0057] In the plasma, the electromagnetic wave (laser beam) can
propagate only when it has a higher frequency than the frequency of
the electron plasma. That is, given that an angular frequency of
the laser beam is .omega..sub.L and an angular frequency of the
electron plasma is .omega..sub.P, the laser beam propagates only in
the low-density plasma region where .omega..sub.L>.omega..sub.P
holds. Here, a plasma electron density NE in which
.omega..sub.L=.omega..sub.P holds is called critical density
Nc.
[0058] When the laser beam is applied to the solid target, since
there is plasma that is spouted and expands from the target
surface, the laser beam propagates from the region with lower
plasma density to the region with higher plasma density while being
absorbed, and reflected at the critical density region. That is,
the laser beam is absorbed in both optical paths to and from the
critical density region in the plasma. Therefore, as the critical
density is higher, the energy can be absorbed by the higher density
plasma, and simultaneously, the risk becomes greater that the
low-temperature high-density plasma region causing debris
generation is produced.
[0059] The critical density Nc is expressed by the following
equation.
N.sub.C(cm.sup.-3)=1.11.times.10.sup.13/.lamda..sup.2
Where .lamda. represents the wavelength of the laser beam.
[0060] FIG. 3 shows wavelengths and critical density of a CO.sub.2
laser and an Nd:YAG laser. The CO.sub.2 laser has the wavelength
.lamda. of the output laser beam an order of magnitude greater than
that of the Nd:YAG laser, and the critical density N.sub.C is two
orders of magnitude less. Consequently, as shown in FIG. 4, the
laser beam outputted from the CO.sub.2 laser is reflected at the
high-temperature low-density plasma region, which is significantly
apart from the target surface. In FIG. 4, the horizontal axis
indicates the plasma electron density NE corresponding to the
distance from the target surface. Further, regarding the Nd:YAG
laser, the cases of fundamental wave .omega. (wavelength 1064 nm)
and the second harmonic wave 2.omega. (wavelength 532 nm) are
shown.
[0061] Using the CO.sub.2 laser as the drive laser, generation of
the high-temperature low-density plasma region as a heat source
that does not contribute to EUV light generation but produces
debris is suppressed. Thereby, the melting of the solid target
surface does not make progress and the neutral particles emitted
from the target and deposited on the reflecting surface of the EUV
collector mirror are significantly reduced. On the other hand, also
fast ions are emitted from the plasma, and the multilayer film
formed on the reflecting surface of the EUV collector mirror is
worn away.
[0062] Furthermore, when solid tin (Sn) or lithium (Li) is used as
the target, the number of neutral particles generated from the
target is much smaller. Accordingly, it has been verified that the
amount of neutral particles deposited on the reflecting surface of
the EUV collector mirror (deposition amount) and the amount of
multilayer film worn away from the reflecting surface of the EUV
collector mirror (sputtering amount) can be balanced, or the
deposition amount can be made smaller than the sputtering amount
under predetermined conditions. Thereby, the problem that debris is
deposited on the reflecting surface of the EUV collector mirror can
be solved.
[0063] The conditions therefor are principally determined by
intensity and/or pulse width of the excitation laser beam generated
by the CO.sub.2 laser. Specifically, it is desirable that the
intensity of the excitation laser beam is set to 3.times.10.sup.9
W/cm.sup.2 to 5.times.10.sup.10 W/cm.sup.2, more preferably,
5.times.10.sup.9 W/cm.sup.2 to 3.times.10.sup.10 W/cm.sup.2.
Further, it is desirable that the pulse width of the excitation
laser beam is set to a relatively short value as about 10 ns to 15
ns.
[0064] In the intensity of the excitation laser beam, the upper
limit is set such that the melting region on the target surface may
not be unnecessarily expanded due to excessive heat application to
the target, and thereby, production of debris can be suppressed. On
the other hand, since the intensity of the excitation laser beam is
greatly affected on the EUV conversion efficiency (CE), the lower
limit is set such that the EUV conversion efficiency may be secured
above a certain level. The relationship between the excitation
laser beam intensity and the EUV conversion efficiency is also
disclosed in Hansson et al., "LPP EUV Source Development for HVM",
SPIE, Vol. 6151, No. 61510R (February, 2006).
[0065] Here, laser beam intensity is expressed by the following
expression.
Laser beam intensity(W/cm.sup.2)=Laser beam energy(J)/{Pulse
width(s)Spot area(cm.sup.2)}
[0066] In the embodiment the collection diameter of the laser beam
is about 100 .mu.m and the spot area is about 7.85.times.10.sup.-5
cm.sup.2, and the laser beam energy is determined to adapt the
conditions. For example, when the pulse width of the excitation
laser beam is set to 12.5 ns, the laser beam energy is about 30
mJ.
[0067] Next, a laser beam application experiment when solid tin is
used as the target will be explained.
[0068] FIG. 5 is a plan view showing an arrangement of detectors in
a laser beam application experiment. In this experiment, a focusing
lens is used as the laser beam focusing optics 14 shown in FIG. 2.
Defining that the incident direction of the excitation laser beam 2
to the target 1 corresponds to a reference angle 0.degree., three
mirror damage detectors 21 are respectively located in three
positions counterclockwise at angles of 22.5.degree., 45.degree.,
75.degree. and three ion detectors 22 are respectively located in
three positions clockwise at angles of 22.5.degree., 45.degree.,
75.degree. when seen from vertically above.
[0069] On the assumption that the amounts of neutral particles and
the amounts of ions emitted from the plasmarized tin target 1 are
symmetric with respect to the reference angle, the amounts of
neutral particles and the amounts of ions can be measured at the
same time in the positions at the three angles with respect to the
reference angle. Therefore, the total change in film thickness,
i.e., the rate of deposition or sputtering if the EUV collector
mirrors are located in the positions can be obtained.
[0070] FIG. 6 shows results of a laser beam application experiment
when a tin plate having a thickness of 1 mm is used as the target.
In FIG. 6, the horizontal axis indicates the angle with respect to
the reference angle, and the vertical axis indicates the rate of
deposition or sputtering, i.e., the change in film thickness per
energy of EUV light (wavelength 13.5 nm, 2% BW, 2.pi.sr) generated
in the plasma.
[0071] As the drive lasers, the Nd:YAG laser (wavelength 1.06
.mu.m) and the CO.sub.2 laser (wavelength 10.6 .mu.m) are used.
Conventionally, when the tin plate is used as the target, the
deposition amount of debris is very large. Actually, as shown in
FIG. 6, when the Nd:YAG laser is used, the film thickness
significantly increases in total (deposition). On the other hand,
when the CO.sub.2 laser is used, the film thickness changes little
and slightly decreases (sputtering).
[0072] FIG. 7 shows results of a laser beam application experiment
when a tin foil having a thickness of 15 .mu.m is used as the
target. In FIG. 7, although the amount of change in film thickness
is much smaller compared to the case where the tin plate is used as
shown in FIG. 6, the results are similar in that the film thickness
increases when the Nd:YAG laser is used and the film thickness
decreases when the CO.sub.2 laser is used.
[0073] Referring to FIG. 1 again, the number of layers of the
multilayer film constituting the reflecting surface of the EUV
collector mirror 15 is about 250 at the maximum in the current
technology, and the film can be used until the thickness of about
1.6 .mu.m is worn away in that case. On the other hand, as
explained above, when tin is deposited on the reflecting surface of
the EUV collector mirror in a thickness of about 0.75 nm, the
reflectance of the EUV collector mirror 15 is about 10% reduced and
the mirror becomes unusable.
[0074] Accordingly, the control unit 30 obtains the amount of
neutral particles deposited on the reflecting surface of the EUV
collector mirror 15 (deposition amount) based on the detection
result (output signal) of the mirror damage detector 21 and the
amount of multilayer film worn away from the reflecting surface of
the EUV collector mirror 15 (sputtering amount) based on the
detection result (output signal) of the ion detector 22, and
thereby, adjusts the pulse width and/or energy of the excitation
laser beam radiated from the drive laser 13 such that the
deposition amount and the sputtering amount are balanced or the
deposition amount is slightly smaller than the sputtering
amount.
[0075] Thus, the deposition of debris on the reflecting surface of
the EUV collector mirror 15 can be prevented and the life of the
EUV collector mirror 15 can be extended. In this regard, it is
desirable that the control unit 30 holds the EUV light intensity at
the light emission point substantially constant based on the
detection result (output signal) of the EUV light detector 24.
[0076] Next, forms of targets to be used in the extreme ultra
violet light source according to the embodiment will be explained.
In the embodiment, targets having various forms such as wires,
tapes, rods, plates or solid droplets can be used.
[0077] FIG. 8 shows a state in which the excitation laser beam is
applied to a wire target, where FIG. 8 (a) is a side view and FIG.
8 (b) is a plan view. FIG. 9 shows a state in which the excitation
laser beam is applied to a tape target, where FIG. 9 (a) is a side
view and FIG. 9 (b) is a plan view.
[0078] FIG. 10 shows a target supply unit for supplying the wire
target or tape target. In the target supply unit 11, a wire/tape
feed mechanism 11a including a feed reel and a motor for feeding
the wire target or tape target is provided. Further, in the target
collection unit 16, a wire/tape take-up mechanism 16a including a
take-up reel and a motor for taking up the wire target or tape
target is provided. In this example, the wire/tape feed mechanism
11a and the wire/tape take-up mechanism 16a constitute part of the
target supply unit.
[0079] Under the control of the control unit 30 (FIG. 1), the
wire/tape take-up mechanism 16a turns the take-up reel for taking
up the target and the wire/tape feed mechanism 11a turns the feed
reel while applying back tension thereto, and thus, a new part of
the target is supplied to a predetermined position within the
vacuum chamber 10 and the part that has been applied with the
excitation laser beam is collected. The target collected from the
vacuum chamber 10 by the target collection unit 16 may be returned
to the target supply unit 11 by providing the target circulation
unit 17 as shown in FIG. 1.
[0080] FIG. 11 shows a state in which the excitation laser beam is
applied to a rod target, where FIG. 11 (a) is a side view and FIG.
11 (b) is a plan view. FIG. 12 shows a target supply unit for
supplying the rod target. The target supply unit 11 and the target
collection unit 16 are provided with a rod transport mechanisms 11b
and 16b that transport the rod target by holding both ends of the
rod target and moving it in parallel (vertically up and down move)
while turning it. In this example, the rod transport mechanisms 11b
and 16b constitute part of the target supply unit. Under the
control of the control unit 30 (FIG. 1), the rod transport
mechanisms 11b and 16b vertically move the rod target while turning
it, and thereby, the excitation laser beam is constantly applied to
new parts of the rod target.
[0081] FIG. 13 is a side view showing an internal structure of the
EUV light source apparatus using a solid droplet target. A target
nozzle 12 is connected to the target supply unit 11. The target
supply unit 11 includes a position adjustment mechanism for
adjusting the fall position of the target 1, and supplies the
target 1 to the predetermined position within the vacuum chamber 10
via the target nozzle 12 while adjusting the fall position of the
target 1. Further, plural target supply units 11 may be provided
according to the supply speed of the solid droplet target.
[0082] The target collection unit 16 includes a position adjustment
mechanism for adjusting the position of collecting of the target 1,
and collects the target that has not been plasmarized while
adjusting the position of collecting the target. The collected
target may be returned by the target circulation unit 17 to the
target supply unit 11 and reused.
[0083] FIG. 14 is a plan view showing an internal structure of the
EUV light source apparatus using a plate target. In this example,
the plate target has a disc shape, and the excitation laser beam is
applied to the circular surface. The EUV light source apparatus has
a plate turning/moving mechanism 41 that turns the plate target
and/or moves the plate target in parallel. The plate turning/moving
mechanism 41 moves the plate target in parallel from bottom to top
in the drawing while turning it, so that the excitation laser beam
is constantly applied to new parts of the plate target.
[0084] In the EUV light source apparatus shown in FIG. 14, the area
of an EUV collector mirror 15a is smaller than that of the EUV
collector mirror 15 shown in FIG. 1, so that the EUV light
reflected by the EUV collector mirror 15a is not applied to the
plate target. Thereby, the collection solid angle of the EUV
collector mirror 15a becomes smaller.
[0085] Next, the second embodiment of the present invention will be
explained.
[0086] FIG. 15 shows a configuration of an EUV light source
apparatus according to the second embodiment of the present
invention. The EUV light source apparatus according to the
embodiment further includes electromagnet coils 51 and 52 as a
magnetic field generating unit for generating a magnetic field when
applied with current, and an electromagnetic power supply 53 that
supplies current to the electromagnet coils 51 and 52 in addition
to the EUV light source apparatus shown in FIG. 13, and includes a
control unit 30a that controls the entire EUV light source
apparatus including the electromagnetic power supply 53 in place of
the control unit 30.
[0087] FIG. 16 is a perspective view of the electromagnet coils
shown in FIG. 15. As shown in FIG. 16, the electromagnet coils 51
and 52 respectively have cylindrical shapes and are located such
that the central axes of the two coils are in line to form a pair
of mirror coils. Further, the electromagnet coils 51 and 52 are
located such that the central axes may pass through the light
emission point. The central axes of the electromagnet coils 51 and
52 may be perpendicular to the laser beam 2, or may not be
perpendicular to the laser beam 2 unless they cut across the
collection optical path of the EUV light. Current is supplied to
the electromagnet coils 51 and 52 in a direction shown by arrows in
FIG. 16.
[0088] The control unit 30a shown in FIG. 15 controls the timing at
which the driver laser 13 generates the excitation laser beam, the
timing at which the target supply unit 11 supplies the target, the
timing at which the electromagnetic power supply 53 flows current
through the electromagnet coils 51 and 52, and so on.
[0089] In the embodiment, a mirror magnetic field is formed by the
electromagnet coils 51 and 52 with the generated plasma 3 in
between, the light emission point of the EUV light is placed in the
magnetic field, and positively charged ions emitted from the plasma
3 are trapped around the light emission point.
[0090] FIG. 17 is a diagram for explanation of a principle of
trapping ions by using a mirror magnetic field. In FIG. 17,
defining that the axis of the electromagnet coils 51 and 52 is the
Z-axis, the intensity of the magnetic field along the Z-axis is
shown by a solid line and the lines of magnetic force are shown by
broken lines. As shown in FIG. 17, when the electromagnet coils 51
and 52 are located such that the central axes thereof are aligned
and current in the same direction is flown in the electromagnet
coils 51 and 52, a magnetic field is formed in which the magnetic
flux density is higher near the electromagnet coils 51 and 52 and
the magnetic flux density is lower between the electromagnet coils
51 and 52.
[0091] A positive ion generated near the Z-axis and having a
velocity "v" in a direction perpendicular to the Z-axis is subject
to a force "F" in the tangential direction of a circle centered at
the Z-axis, and trapped near the Z-axis. FIG. 18 shows tracks of
ions when the interior of the EUV light source apparatus shown in
FIG. 15 is seen from above. In FIG. 18, the tracks of ions around
the Z-axis are enlarged. As shown in FIG. 18, the ion generated
near the Z-axis and having a velocity in a direction perpendicular
to the Z-axis is subject to a force perpendicular to the velocity
within a plane perpendicular to the Z-axis, rotated, and trapped
near the Z-axis.
[0092] On the other hand, an ion having a velocity along the Z-axis
is hardly acted by the magnetic field, but the ion traveling in the
Z-axis direction does not collide with the EUV collector mirror 15.
Further, the ion having the velocity component along the Z-axis can
be turned around near the light emission point when the magnetic
field is generated to satisfy the following expression (1)
sin .theta.>(B.sub.MIN/B.sub.MAX).sup.1/2 (1)
Where B.sub.MIN is the lowest magnetic flux density between the
electromagnet coils 51 and 52, B.sub.MAX is the highest magnetic
flux density near the electromagnet coils 51 and 52, and .theta. is
an angle formed by the velocity vector of an ion in a position
where the magnetic flux density is B.sub.MIN and the Z-axis. The
B.sub.MIN and B.sub.MAX are determined according to the shapes of
the electromagnet coils 51 and 52.
[0093] The ion that satisfies the expression (1) is confined in a
potential valley by the magnetic field generated by the
electromagnet coils 51 and 52, while the ion that does not
satisfies the expression (1) is not confined by the magnetic field.
However, the electromagnet coils 51 and 52 and the EUV collector
mirror 15 may be located such that the ion that does not satisfies
the expression (1) may not collide with the EUV collector mirror
15.
[0094] The magnetic field can trap electrons radiated from the
plasma around the light emission point as well as the positive
ions, and thus, the diffusion rate of positive ions that tend to
diffuse due to the coulomb forces between the positive ions can be
reduced by the coulomb forces between the positive ions and the
electrons. Therefore, by trapping electrons around the light
emission point, Larmor radii of positive ions can be reduced and
the positive ions can be easily trapped around the light emission
point.
[0095] Referring to FIG. 15 again, the control unit 30a controls
the target supply unit 11 to repeatedly supply the target 1 at
predetermined intervals, and controls the driver laser 13 to
generate the laser beam 2 in a period in which the target 1 is
supplied. Concurrently, the electromagnet coils 51 and 52 may
generate a stationary magnetic field, or generate a pulse magnetic
field according to the generation of the laser beam 2. When a pulse
magnetic field is generated, the control unit 30a controls the
electromagnetic power supply 53 to generate the pulse magnetic
field with the electromagnet coils 51 and 52 in a period from the
time when the generation of the laser beam 2 is started to the time
when generation of the laser beam 2 is stopped and the ions lose
kinetic energy to the degree that the ions do not cause damage on
the EUV collector mirror 15.
[0096] Further, the EUV light source apparatus includes a mirror
damage detector 21 located near the EUV collector mirror 15 for
monitoring the change in film thickness of the EUV collector mirror
15 and detects the change in thickness of a sample film (a film for
measurement) due to neutral particles and ions emitted from the
plasma. Accordingly, based on the detection result (output signal)
of the mirror damage detector 21, the control unit 30a can control
the electromagnetic power supply 53 to adjust the magnetic field
intensity such that the amount of neutral particles deposited on
the sample film (deposition amount) and the amount of components
worn away from the sample film (sputtering amount) may be balanced,
or the deposition amount may be slightly smaller than the
sputtering amount.
[0097] Next, the third embodiment of the present invention will be
explained.
[0098] FIG. 19 is a perspective view of an electromagnet coil in an
EUV light source apparatus according to the third embodiment of the
present invention. The component elements other than the
electromagnet coil are the same as those in the EUV light source
apparatus according to the second embodiment of the present
invention as shown in FIG. 15.
[0099] As shown in FIG. 19, the electromagnet coil 54 has a shape
along the seams of a baseball, and generally called a baseball
coil. The electromagnet coil 54 is provided so as to encompass the
plasma 3 to be generated and wrap around the light emission point
of EUV light, and thus, the light emission point of EUV light is
located within the magnetic field. Especially, in the embodiment,
the electromagnet coil 54 is provided such that the light emission
point of EUV light is at the center of the ball shape. The baseball
magnetic field generated by the electromagnet coil 54 traps
positively charged ions emitted from the plasma generated at the
light emission point around the light emission point.
[0100] FIG. 20 is a diagram for explanation of a principle of
trapping ions using a baseball magnetic field. In FIG. 20, the
intensity of the magnetic field along the X-axis and the Y-axis
passing through the center of the electromagnet coil 54 and
perpendicular to each other is shown by a solid line. As shown in
FIG. 20, when current is flown in the electromagnet coil 54, a
magnetic field in which the magnetic flux density is lower at the
central part of the electromagnet coil 54 and the magnetic flux
density increases in every direction from the central part toward
the periphery of the ball configuration is formed.
[0101] Therefore, an ion having a velocity in a direction away from
the central part of the electromagnet coil 54 is turned around by
the strong intensity near the end surface of the nearly spherical
shell-shaped space surrounded by the electromagnet coil 54, and the
ion can be trapped near the center of the electromagnet coil 54
(around the light emission point).
* * * * *